Improving performance of thermally activated delayed fluorescence emitter by extending its LUMO distribution

An optimized compound 9-(9,9-dimethylacridin-10(9 H)-yl)-6 H-benzo[c]ch-romen-6-one(MAB) was designed and synthesized based on our previously reported TADF emitter 6-(9,9-dimethylacridin-10(9 H)-yl)-3-methyl-1 H-isochromen-1-one(MAC) to further improve the performance of thermally activated delayed fluorescence(TADF)emitters. With the additional phenyl in coumarin-contained plane, MAB possesses an extended distribution of the lowest unoccupied molecular orbitals(LUMO), and thus realizes reduced electron exchange between the frontier molecular orbitals and a stretched molecular dipole moment compared with MAC. MAB based organic light-emitting diode(OLED)exhibits a remarkable maximum external quantum efficiency(EQE) of 21.7%, which is much better than the maximum EQE of MAC-based OLED with a value of 12.8%. Our work proves that extending the distribution of LUMO is a simple but effective method to improve the efficiency of TADF emitter.

Novel donor-spacer-acceptor compound as the multifunctional component of exciplexes for efficient organic light-emitting diodes

By connecting tercarbazole(3 Cz)and triphenyltriazine(TRz)units with a diphenyl ether group,we designed and synthesized a new donor-spacer-acceptor structure molecule9’-(4-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenoxy)phenyl)-9’H-9,3’:6’,9"-tercarbazole(3 Cz-o-TRz)as the multifunctional component of exciplex emitters.As expected,the intermolecular charge-transfer transition is dominant for3 Cz-o-TRz because of the space-enough and conjugation-forbidden linkage of the diphenyl ether group,increasing the intrinsic characteristics of the 3 Cz and TRz moieties in its single-molecule state.Accordingly,three common electron acceptors(4,6-bis(3,5-di(pyridin-3-yl)phenyl)-2-methylpyrimidine(B3 PyMPm),4,6-bis(3,5-di(pyridin-4-yl)phenyl)-2-methylpyrimidine(B4 PyMPm),and(1,3,5-triazine-2,4,6-triyl)tris(benzene-3,1-diyl)tris(diphenylphosphineoxide)(POT2 T))and three common electron donors(4,4’-(cyclohexane-1,1-diyl)bis(N,N-di-p-tolylaniline)(TAPC),tris(4-(10 H-phenoxazin-10-yl)phenyl)amine(TPA-PXZ),and N1-phenyl-N4,N4-bis(4-(phenyl(m-tolyl)amino)phenyl)-N1-(m-tolyl)benzene-1,4-diamine(m-MTDATA))were chosen to construct six exciplex emitters with 3 Cz-o-TRz.In the organic light-emitting diodes(OLEDs),the emission spectra of these exciplex emitters could be gradually modulated from 510 to 590 nm.In addition,TAPC:3 Cz-o-TRz-and 3 Cz-o-TRz:PO-T2 T-based OLEDs achieved desirable performance with maximum external quantum efficiencies(EQEs)around 12%.Moreover,a simple tandem OLED containing TAPC:3 Cz-o-TRz and 3 Czo-TRz:PO-T2 T emitters realized optimal performance with an ultralow turn-on voltage of 2.4 V and a maximum EQE of14.1%.These results indicate the great potential of the donorspacer-acceptor structure compounds in developing exciplex emitters.

Improving Efficiency of Red Thermally Activated Delayed Fluorescence Emitter by Introducing Quasi-Degenerate Orbital Distribution

Red thermally activated delayed fluorescence(TADF)emitters are essential for the development of organic light-emitting diodes(OLEDs).To better understand and develop red TADF emitters,herein,the effect of quasi-degenerate orbital distribution was inves-tigated on their performance.Two red TADF emitters 10,10'-(11,12-difluorodibenzo[a,c]phenazine-3,6-diyl)bis(10H-phenoxazine)(DPXZ-BPF)and 10,10',10"-(12-fluorodibenzo[a,c]phenazine-3,6,11-triyl)tris(10H-phenoxazine)(TPXZ-BPF)were constructed by combining two or three phenoxazine(PXZ)donor units with a rigid coplanar acceptor core.As expected,a couple of PXZ units can induce quasi-degenerate orbital distribution by virtue of the high steric hindrance between each donor unit and acceptor core,which enables multiple excited states engaged in the reverse intersystem crossing(RISC)process.Consequently,DPXZ-BPF and TPXZ-BPF exhibit fast RISC rates(k_(RISCs))of 1.07×10^(6) s^(-1) and 1.29×10^(6) s^(-1) and high PLQYs of 74.6% and 81.1%,respectively.And the higher kRISC and PLQY of TPXZ-BPF are mainly attributed to the additional quasi-degenerate orbitals,which further enhance the tri-plet RISC process and the singlet radiative process.As a result,TPXZ-BPF-based OLEDs achieved improved efficiencies with a red-shifted emission compared with DPXZ-BPF-based OLEDs.These results demonstrated the tremendous potential of introducing quasi-degenerate orbital distribution in developing red TADF emitters.

Novel bipolar host for highly efficient green,yellow,orange,red and deep-red phosphorescent organic light-emitting devices

A novel bipolar host tris(4-(pyrimidin-5-yl)phenyl)amine(TPMTPA) constructed by incorporating triphenylamine as the electron-donating core and pyrimidine as the electron-accepting peripheries was designed and synthesized.TPMTPA achieves excellent bipolar charge transport properties and has high enough triplet energy level to sensitize green,yellow,orange,red and deep-red phosphors.By using TPMTPA as a host,high performance green,yellow,orange,red and deep-red phosphorescent organic light-emitting devices(PhOLEDs) were demonstrated with maximum external quantum efficiencies of 20.4%,17.6%,15.1%,15.3%and 15.7%respectively.These results suggested that TPMTPA is a versatile high performance host for PhOLEDs of different emission colors.